...
Well, I of course in saying this absolutely do not condone the murder of any one but am simply answering the question
Implantation of a common but lethal contagious disease, if it didn't require injection people would assume the person got the disease naturally.
Also, gas, particularly carbon monoxide because it's quite common for a household to have a CO leak.
If you could go to space, knowing the dangers and training, would you do it?